The premium on weight of a spacecraft limits the amount of fuel that can be carried into space. The success of any space mission depends on providing adequate power for the functioning of the spacecraft. Most spacecraft rely on solar panels to provide power. However, solar panels are idle during night periods. Fuel cells regenerated by solar cells can provide power during the night periods but are limited by the low efficiency of solar cells.
Waste heat or solar generated heat is also available on a spacecraft. The heat can be converted to electricity. Conventionally heat from Radioactive Thermal Generators, (RTG) is fed to thermocouples or thermionic devices to produce electrical power. However, these conversion methods are very inefficient because devices only provide a single stage of electrical generation in the millivolt range and are limited in conversion efficiency.
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